Oxygen atmosphere found on Saturn's moon

WASHINGTON: Astronomers have announced that an oxygen atmosphere has been found on Saturn's second largest moon, Rhea.

At about 527,000 kilometres from Saturn, Rhea orbits inside the planet's magnetic field. Rhea's oxygen atmosphere is maintained by the ongoing chemical breakdown of water ice on the moon's surface, driven by radiation from Saturn's magnetosphere.

NASA's Cassini spacecraft, which has been orbiting the Saturnian system since 2004, detected the oxygen atmosphere around Rhea during a close flyby of the icy moon in March.

Cassini's data show that molecular oxygen (O2) forms inside the moon's surface ice when water molecules (H2O) are split by energetic ions, a process known as radiolysis. The oxygen then gets ejected from the surface ice and captured by Rhea's gravity to form the atmosphere.

"The major implication of this finding at Rhea is that oxygen atmospheres at icy moons, until now only detected at Europa and Ganymede, may in fact be commonplace around those irradiated icy moons throughout the universe with sufficient mass to hold an atmosphere," National Geographic News quoted study leader Ben Teolis of the Southwest Research Institute, as saying.

"A loose analogy might be carbon dioxide dissolved, or trapped, in a carbonated beverage, except here we are not talking about liquid water but rather frozen ice at extremely low temperatures," said Teolis.

The amount of oxygen gas produced per second across Rhea's surface weighs about 130 grams, the study team reported.

Cassini also identified the distinctive chemical fingerprint of carbon dioxide in Rhea's atmosphere, indicating the presence of carbon on the moon's surface.

"You would expect a very small amount of gas [around an ice moon], but the fact that there is enough to be measurable is what is surprising and indicates that the energetic processes that must be occurring are more widespread than previously thought," said Robert Carlson, of the NASA's Jet Propulsion Laboratory, California.

The new study may have scientists looking with fresh eyes at Rhea-like moons around other gas giant planets.

"The discovery of Rhea's atmosphere is extremely fortuitous, as it will allow us to anticipate what we might expect to find at Jupiter's moons and design the spacecraft instruments accordingly," said Teolis.